Conventionally, piezoelectric elements have the characteristic of contracting or expanding as a function of a direct voltage applied to them or of a direct voltage established across them. The practical implementation of actuators using piezoelectric elements may be suitable when the actuator has to perform quick and/or frequent movements. Among other things, the piezoelectric element may be used as an actuator in fuel injectors for internal combustion engines. For certain applications it may be necessary that different degrees of expansion or, if needed, varying degrees of expansion be induceable in the piezoelectric element as precisely as possible; for example, when the piezoelectric element is used as an actuator in a fuel injection system. Through direct or indirect transmission to a control valve, different degrees of expansion of the piezoelectric element correspond to the displacement of an actuator, for example, a nozzle needle. The displacement of the nozzle needle results in the opening of injection orifices. The duration of the opening of the injection orifices corresponds to a desired injected quantity as a function of a free cross section of the orifices and an applied pressure. The control valve which controls the movement of the nozzle needle need not be triggered directly, but may be triggered via a hydraulic coupler starting at the piezoelectric element.
The piezoelectric element, together with the hydraulic coupler, the adjoining control valve and the nozzle needle, forms a complex spring-mass system. No excessive oscillations are to be induced in the spring-mass system by the triggering, because this would affect the desired injected quantity. Excitation of oscillations of the piezoelectric element may thus not be arbitrarily short.
Assuming a predefined voltage level, there is a lower time threshold at which a trigger duration, composed of a charging operation and a holding operation, may not be shortened any further without causing oscillations of the spring-mass system.
A conventional operating mode using a specific trigger cycle may make a non-oscillating deflection of piezoelectric elements possible via the modification of the trigger duration; however, the lower time threshold as a limit remains. In the case of a technical requirement for arbitrarily small injected quantities under an extremely high rail pressure in fuel injection systems, non-oscillating triggering is no longer possible using the conventional methods.